restful.rb

module RESTful
  extend ActiveSupport::Concern

  def resource_name
    @resource_name ||= controller_name.singularize
  end 

  def model_class
    @model_class ||= resource_name.camelize.constantize
  end 

  def param_sym
    @param_sym ||= resource_name.to_sym
  end 

  def index
    instance_variable_set("@#{resource_name.pluralize}", model_class.scoped)
    respond_with instance_variable_get("@#{resource_name.pluralize}")
  end 

  def show
    instance_variable_set("@#{resource_name}", model_class.where(id: params[:id]).first)
    respond_with instance_variable_get("@#{resource_name}")
  end 

  def create
    instance_variable_set("@#{resource_name}", model_class.new(params[param_sym]))
    flash[:notice] = "#{model_class.class_name} was successfully created." if instance_variable_get("@#{resource_name}").save
    respond_with instance_variable_get("@#{resource_name}")
  end 

  def update
    instance_variable_set("@#{resource_name}", model_class.where(id: params[:id]).first)
    flash[:notice] = "#{model_class.class_name} was successfully updated." if instance_variable_get("@#{resource_name}").update_attributes(params[param_sym])
    respond_with instance_variable_get("@#{resource_name}")
  end 

  def destroy
    instance_variable_set("@#{resource_name}", model_class.where(id: params[:id]).first)
    flash[:notice] = "#{model_class.class_name} was successfully deleted." if instance_variable_get("@#{resource_name}").destroy
    respond_with instance_variable_get("@#{resource_name}")
  end
end

users_controller.rb

class UsersController < ApplicationController
  include ::RESTful

  respond_to :json
end